Field of the Invention
The invention relates to the manufacture of containers, such as bottles or pots, obtained by forming, and more specifically by blow molding or stretch blow molding, starting from blanks made of thermoplastic material.
Description of the Related Art
To manufacture a container according to the blow-molding technique, the first step is to heat a blank (regardless of whether it is a preform or an intermediate container that has undergone a first blow-molding operation starting from a preform) to a temperature that is higher than the glass transition temperature of the constituent material of the blank. The blank is then inserted into a mold, and then the blow molding of the blank is carried out by injecting a gas (such as air) therein under high pressure (in general more than 20 bar).
The stretch-blow-molding technique consists, in addition to the blow molding, in stretching the blank by means of a sliding rod so as to minimize offsetting of the container and to make the distribution of the material as uniform as possible.
A container comprises a side wall (also called a body), a neck that extends from an upper end of the body, and a bottom that extends from a lower end of the body, opposite the neck. The bottom of the container defines a base by which the container can rest on a flat surface (such as a table).
The mold comprises a wall that defines a cavity intended to impart its shape to the body of the container. This cavity is closed, at a lower end, by a mold bottom intended to impart its shape to the bottom of the container.
One of the primary objects targeted today by manufacturers is the decrease in the quantity of material used, which is reflected by a reduction in the weight of the containers, regardless of the destination of the former (still liquids, carbonated liquids, liquids introduced hot into the containers). In return for this lightening of the containers, an attempt is made to increase their rigidity by means of artifacts linked either to the method of manufacturing or to the design, because the structural rigidity associated with just the bi-orientation (axial and radial molecular orientation in relation to the longitudinal central axis of the container) appears inadequate.
It even happens that certain specifications (in particular for hot filling applications or HR—heat resistant-applications) prescribe the reduction in weight and the increase in the structural rigidity of the container at the same time, which increases the design difficulties of the former. In the HR applications, the structural rigidity of the container can be increased thermally by means of a heat-setting (in English, heat set) of the material, consisting in keeping the container in contact with the heated wall of the mold, which increases the crystallinity of the material.
Concerning more particularly the bottom of the container, its structural rigidity can also be increased (or controlled) mechanically by means of a specific local distribution of the material (leading to an additional stretching of the former), by means of a mold provided with a stationary wall bearing the impression of the body of the container, and a mold bottom bearing the impression of the bottom of the container, with this mold bottom being mounted to move in relation to the wall. The container is first blow-molded beyond its final shape, in a lower position of the mold bottom, and then the mold bottom is moved toward an upper position corresponding to the final shape of the container.
This technique, referred to as “boxing,” illustrated in the French patent application FR 2 938 464 (SIDEL PARTICIPATIONS) or its U.S. equivalent US 2012/031916, makes it possible to improve the mechanical strength of the bottom of the container, in particular in the area of the base.
In such a mold, a gap is provided between the mold bottom, mounted on a guide jack, and the wall, to maintain between these two parts operational play that has two purposes: on the one hand, to make possible the movement without wedging of the mold bottom in relation to the wall; on the other hand, to form a decompression air vent making it possible to evacuate the air that is trapped between the mold and the container during blow molding.
This technique is satisfactory but can be improved upon.
Actually, the operational play between the mold and the mold bottom cannot be less than the guiding precision of the jack, which is on the order of several tenths of millimeters for travel on the order of 20 to 40 mm.
In other words, this operational play is of the same order of magnitude as the thickness of the material of the final container. The material consequently has a tendency, during blow molding, of flowing into the gap when the mold bottom is in the lower position. The thus pinched material forms, when the mold bottom is moved toward its upper position, a thin bead of material that remains on the final container. This bead, forming a projection on the base of the container, is detrimental to the stability of the former.
The need for stability is particularly critical for certain containers whose bottoms can deform under the effect of significant bending and buckling stresses; cf., for example, the container described in the document EP 2 711 152 (SIDEL PARTICIPATIONS), whose bottom comprises a mechanically reversible membrane for compensating a decrease in volume of the contents accompanying the cooling of the former and putting the container under pressure for increasing its rigidity.
A first solution can consist in trimming the container, by cutting or by abrasion. This solution is not realistic on the industrial scale, however, taking into account production rates (several tens of thousands of containers per hour and per blow-molding machine).
A second solution can consist in adding to the jack a precision guiding device (for example with balls), in such a way as to reduce the operational play between the mold bottom and the wall of the mold. This solution, however, runs up against practical difficulties, because the space requirement of the guiding device would make it necessary to modify by depth the structure of the mold, whereas the space is counted all around the former, taking into account in particular the presence of ducts and connections that are necessary for the circulation of temperature-regulating fluids (heating and/or cooling) in the wall of the mold.
A third solution can consist in anticipating the command for raising the mold bottom, so that the material does not have time to slip back between the wall of the mold and the mold bottom. Under these conditions, however, the material that is intended for the bottom of the container is insufficiently stretched, and the base proves to be poorly formed, which reduces the advantage of the boxing.
In addition, the reduction in the operational play between the mold bottom and the wall of the mold is likely to impede the evacuation of the air that is present in the mold, with a risk of malformation of the container (with an equivalent cycle time) or of a reduction in the cycle time (with an equivalent quality of the container).